1. Field of the Invention
The present invention relates to a magneto-optical recording medium, wherein information is recorded and reproduced using a laser beam, and more specifically, to a magneto-optical recording medium and a magneto-optical reproducing method, which are capable of realizing high-density recording and super-resolution reproduction.
2. Related Background Art
As a high-density recording system which is rewritable, a magneto-optical recording system has been receiving a lot of attention, wherein information is recorded by writing magnetic domains in a magnetic thin film using the thermal energy of a semiconductor laser beam, and the recorded information is read out using a magneto-optical effect. In recent years, the demand has been increasing to enhance the recording density of this magneto-optical recording medium for further increasing its storage volume.
The line recording density of an optical disc, such as, a magneto-optical recording medium, largely depends on a laser beam wavelength 1 in the reproducing optical system and the numerical aperture, N.A., of an objective lens. Specifically, since the diameter of a beam is determined when the reproducing light wavelength and the objective lens aperture number are determined, the shortest mark length which can be reproduced is limited to about .lambda./2 N.A.
On the other hand, the track density is mainly limited by crosstalk between adjacent tracks and depends on the diameter of the reproducing beam spot like the shortest mark length.
Accordingly, in order to realize higher-density recording with a conventional optical disc, it is necessary to shorten the laser beam wavelength in the reproducing optical system or increase the numerical aperture, N.A., of the objective lens. However, it is not easy to shorten the laser beam wavelength due to a drop in the efficiency of the element, generation of heat, and the like. On the other hand, when increasing the numerical aperture of the objective lens, the processing of the lens becomes difficult, and further, the distance between the lens and the disc becomes so short that a mechanical problem, such as, collision with the disc, occurs. In view of this, techniques have been developed to improve the structure of the recording medium and the information reading method so as to increase the recording density.
For example, in a magneto-optical reproducing method as disclosed in Japanese Patent Application Laid-open No. 3-93056, a medium structure as shown in FIGS. 1A to 1C has been proposed. FIG. 1A is a sectional view of an optical disc as an example of the super-resolution technique. A substrate 20 is normally formed of a transparent material, such as, glass or polycarbonate. On the substrate 20, an interference layer 34, a reproduction layer 31, an intermediate layer 32, a memory layer 33 and a protective layer 35 are laminated in the order named. An interference layer 34 is provided for enhancing the Kerr effect, and the protective layer 35 is provided for protecting the magnetic layers. Arrows in the magnetic layers each represent the direction of magnetization or atomic magnetic moment in the magnetic film. A light spot irradiates the medium having the reproduction layer, the intermediate layer and the memory layer to form a temperature distribution on the medium. In the temperature distribution, magnetic coupling between the reproduction layer and the memory layer at a high-temperature region is cut off by the intermediate layer having a low Curie temperature, and magnetization of the reproduction layer at the portion where the magnetic coupling was cut off, is aligned in one direction by an external magnetic field, so as to mask a portion of magnetic-domain information of the memory layer within the light spot. In this manner, a signal having a period equal to or smaller than the diffraction limit of light can be reproduced so as to improve the line recording density.
On the other hand, in super-resolution producing methods as disclosed in Japanese Patent Application Laid-open Nos. 3-93058 and 4-255946, a medium formed of a reproduction layer 31, an intermediate layer 32, and a memory layer 33 is used as shown in FIGS. 2A to 2C. Prior to reproducing information, magnetization of the reproduction layer 31 is aligned in one direction by an initializing magnetic field 21 so as to mask magnetic-domain information of the memory layer 33. Thereafter, a light spot 2 irradiates the medium to form a temperature distribution on the medium. In the temperature distribution, the initialized state of the reproduction layer 31 is held in a low-temperature region to form a front mask 4. On the other hand, in a high-temperature region where the temperature is equal to or higher than a Curie temperature Tc2 of the intermediate layer 32, magnetization of the reproduction layer 31 is forcibly oriented in a direction of the reproducing magnetic field 22 so as to form a rear mask 5. Only in a medium-temperature region, is the magnetic-domain information of the memory layer 33 transferred so as to reduce the effective size of the reproducing light spot. By this arrangement, a recorded mark 1 equal to or smaller than the diffraction limit of light can be reproduced so as to improve the line recording density.
On the other hand, in Japanese Patent Application Laid-open No. 6-124500, a magneto-optical recording medium structure has been proposed, as shown in FIGS. 3A to 3C, for providing a super-resolution technique to realize a recording density exceeding the optical resolution of the reproduced signal.
FIG. 3A is a sectional view of an optical disc as an example of the super-resolution technique. Arrows in the magnetic films each represent a direction of the iron family element sublattice magnetization in the film.
The memory layer 42 is a film formed of a material, such as, TbFeCo, DyFeCo or the like, having a large perpendicular magnetic anisotropy. Information is held in the memory layer 42 in the form of magnetic domains, which are directed upward or downward relative to a film surface. The reproduction layer 41 is an in-plane magnetization film at room temperature and becomes a perpendicular magnetization film when increased in temperature to Tl-mask.
When information reproducing light irradiates the disc having the foregoing medium structure from a side of the substrate 20, a temperature gradient at the center of the data track becomes as shown in FIG. 3C. When viewing this from the side of the substrate 20, an isotherm of Tl-mask exists in the light spot as shown in FIG. 3B. As described above, since the reproduction layer 41 is an in-plane magnetization film at a temperature lower than Tl-mask, it does not contribute to the Kerr effect (forming the front mask 4) at that portion so that the recorded magnetic domain held in the memory 42 is masked by the front mask 4. On the other hand, at a portion where a temperature is no less than Tl-mask, the reproduction layer 41 becomes a perpendicular magnetization film, and further, the direction of the magnetization becomes the same as the recorded information due to the exchange-coupling force from the memory layer 42. As a result, the recorded magnetic domain of the memory layer 42 is transferred only to an aperture portion 3, which is smaller than the size of the spot 2 so that super resolution is realized.
In the foregoing known super-resolution techniques, since the front mask 4 at the low-temperature region extends toward the adjacent tracks, those techniques aim to also improve the track density along with the line recording density.
However, in the method disclosed in Japanese Patent Application Laid-open No. 3-93056, although the resolution can be enhanced without reducing signal equality, it is necessary to apply the reproducing magnetic field. Further, in the methods disclosed in Japanese Patent Application Laid-opens Nos. 3-93058 and 4-255946 it is necessary to align the magnetization of the reproduction layer 31 in one direction prior to reproducing information so that an initializing magnet 21 for that purpose should be added to the conventional device. Further, in the super-resolution reproducing method disclosed in Japanese Patent Application Laid-open No. 6-124500, since only the front mask 4 is used, when expanding the mask region for enhancing the resolution, the position of the aperture 3 deviates from the center of the spot causing a deterioration in signal equality.
As described above, the conventional super-resolution reproducing methods include problems such that the resolution can not be increased to a sufficient level, the magneto-optical recording/reproduction apparatus is complicated in structure and is expensive and it is difficult to reduce the size thereof.